A. Field of the Invention
The present invention relates to methods and apparatus for dispersing a fluid into a liquid for the purpose of promoting phase transfer into the liquid. The present invention performs this with high energy efficiency and with greatly improved phase transfer efficiency. The present invention also relates to the production and use of small gas bubbles for purposes other than gas transfer as, for example, the separation of solids from liquid by flotation.
B. Description of the Prior Art
Aeration is the largest energy consumer of the presently used techniques for controlling water pollution. Aerators are extensively used to maintain aerobic conditions for sewage digestion and a host of other biological waste treatment processes. Aeration equipment is often employed to provide mixing in biological reactors and to prevent the settling of solids. A conventional aerator is usually of high capacity (10-100 horsepower) and includes a large, finned, rotating turbine whose diameter might range from 20-60 inches (50-150 centimeters). Such units usually have efficiencies of about 1.5-1.7 pounds of oxygen transferred for each horsepower hour of energy expended. A typical 25 horsepower aerator might transfer about 1000 lbs. of oxygen each day into the surrounding liquid. A more elaborate approach involves forcing the gas, sometimes pure oxygen, through a porous disc immersed in the liquid, creating an exhaust of fine bubbles.
Several advanced aerators have been developed which include specially shaped and finned turbines of smaller size than the conventional rotors mentioned above. Another type incorporates a larger rotor operating close to a stator with air sheared in the space defined between the two opposing pieces of metal. Such aerators have achieved up to about 2.65 lbs. oxygen transferred per horsepower hour, or roughly 65% better energy efficiency than conventional aerators.
The capital cost of aeration equipment is related to the size of the motor used because the motor is the major cost component in most systems. The other major costs are for the installation of the unit and for the supply of electrical power. Any major change in the efficiency of an aerator produces not only operating cost savings by reducing energy consumption during treatment but also substantially decreases capital costs for the equipment by reducing the size of the motor required for a given amount of aeration capacity. An aerator with a two-fold increase in efficiency and a design similar to that of a conventional aerator, would reduce operating costs by almost 40% and capital costs for the system by a factor of 30%.
The major aim in most systems of conventional design is to maximize turbulence (surface aerators) and increase the interfacial surface area between the liquid and gas (assumed to be air or oxygen in most cases). Advanced aerators usually are designed to produce small bubbles as they have large surface areas and slow rise velocities. Such small bubbles remain in contact with the fluid for a long period of time and greatly improve gas to liquid transfer. Generating small bubbles, rather than promoting surface turbulence, is a better approach to more efficient aeration. Small bubbles can also be used for flotation (solid/liquid separation), protein extraction by the concentrating of surface-active solutes in the bubble membrane, or for density-dependent separation. Accordingly, an efficient small-bubble generator could be applied to many uses. However, the energy costs of most small-bubble generators prove prohibitive. It is an object of the present invention to provide an energy efficient small-bubble generator.